Since the earliest times, human beings have wanted to explain the most
unpredictable and disturbing phenomena in the universe. Although the study
of astronomy has been a constant in all civilizations, astronomical events
of a more “unpredictable” nature, such as comets or eclipses, were
considered an “omen of misfortune” and/or “actions of the gods.”
The fall of the Saxon king Harold II in 1066, during the Norman invasion of
William the Conqueror, was attributed to the bad omen from the passage of a
comet (later baptized as “Halley”). And during the battle of Simancas
(Valladolid, Spain) between the troops of León Ramiro II and the Caliph Ad
al-Rahman in 939, a total solar eclipse caused panic among the troops on
both sides, delaying the battle for several days.
How would our ancestors have reacted, then, to the existence in the universe
of objects – so-called black holes – capable of swallowing everything that
fell into them, including light?
While the biggest black holes have been already detected and even
photographed, there is now also feasible evidence – as I show in my
recent study
– for tiny black holes the size of potassium atoms (with a radius of about
0.23 nanometers, equivalent to 0.23 billionth of a meter). These
atomic-sized black holes were formed in the first moments of the Big Bang
and may even comprise the totality of the dark matter of the universe.
Taking photos
In 2019, a collaboration of eight radio telescopes located in different
parts of the world was able to take the first photo of a gigantic black hole
(6.5 billion times more massive than our Sun). It is located about 55
million light years from us (a light-year corresponding to a distance of
about 9.5 trillion kilometers) at the center of the Messier 87 galaxy.
The italics of the word photograph is no coincidence: how can a photograph
be taken of an object that catches light and, therefore, would not be able
to be seen by cameras, which use light to create a picture? The answer is
simple: we are not observing the object itself, but the remains of star that
are being swallowed up by these black holes.
This stellar matter rotates at enormous velocities around the black hole and
its brightness can be detected when it reaches temperatures of the order of
a million degrees celsius. The disk of matter that surrounds the black hole
is called the “accretion disk” and is considered the edge of the black hole
– once it is passed, nothing can escape, something we call an event horizon.
Primordial black holes
Significant parts of the black holes in the universe were formed by the
gravitational collapse of stars consuming all their fuel in their final
stages: these are called “stellar black holes.” Not all stars will turn into
black holes at the end of their lifetime; when the core of a star is less
than two or three solar masses, a stellar black hole cannot be created.
That is, there exists a minimum stellar mass below which a star cannot
collapse into a black hole. As an example, our Sun will never turn into a
black hole at the end of its life, but other massive stars like the red
supergiant Betelgeuse will inevitably become black holes.
There are also other black holes called “primitive” or “primordial” black
holes, which – as their name indicates – were created in the first moments
of the Big Bang, when the universe first began, and can theoretically
possess any mass. They may range in size from a subatomic particle to
several hundred kilometers.
And when it comes to black holes, supermassive ones emit practically no
radiation, while the smallest ones emit the most radiation. But, how is this
phenomenon possible: supermassive black holes that emit practically no
radiation and trap everything, even light?
The answer was provided by physicist Stephen Hawking in the mid-1970s. He
postulated that the quantum effects near the event horizon of a black hole
might produce the emission of particles that could escape from it. That is,
black holes that do not gain mass by any other means will progressively lose
their mass and finally evaporate.
This Hawking radiation is more evident in low-mass black holes: the
evaporation time of a million-solar-mass supermassive black hole is 36×10 to
the power of 91 seconds (much longer than the current age of the universe).
On the other hand, a black hole with a mass equivalent to a 1,000-ton ship
would evaporate in about 46 seconds.
In the last stages of a black hole’s evaporation, they would explode and
generate a huge amount of gamma rays (a radiation even more intense than
X-rays).
Capturing an atomic-sized primordial black hole
So how can atomic-sized holes be evidenced before they evaporate completely?
In the recent study of atomic-sized black holes, an astrophysical scenario
is proposed where one of these tiny black holes is captured by a
supermassive one. As the atomic-sized black hole approaches the event
horizon of the supermassive one, the fraction of Hawking radiation that
might be detected from the Earth gradually decreases, until it reaches the
size of a ray of light.
This beam is compatible with thermal gamma ray bursts (GRBs) already
measured at astronomical observatories. It is these GRBs that constitute an
experimental evidence for such tiny black holes, which are serious
candidates for the dark matter of a yet unexplored and fascinating universe.
Written by Oscar del Barco Novillo, Profesor asociado en el área de Óptica,
Universidad de Murcia.
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